Heavy petroleum fractions, such as vacuum gas oil, or even resids such as atmospheric residuum, may be catalytically cracked to lighter and more valuable products. In the United States, catalytically cracked gasoline is especially valuable. In Europe, middle distillates such as diesel are fuels of significant importance. It is conventional to recover the product of catalytic cracking and to fractionate the cracking products into various fractions such as light gases; naphtha, including light and heavy gasoline; distillate fractions such as cycle oil and heavier fuel oil fractions (HFO).
Environmental regulations, particularly in Europe, are expected to become more stringent in the future. The sulfur content and 85% point of distillate fuels, such as diesel, will require reduction. The sulfur content of the diesel fuel products of this invention is not to exceed 500 ppm by weight or 0.05 wt. %. In order to reduce the 85% point, high molecular weight components, especially naphthenes and aromatics must be hydrocracked. The 85% point is the temperature at which 85% of the volume of a feed such as a middle distillate has been removed by distillation. If few long chain molecules are present in the feed, the 85% point will be lower than if many long chain molecules are present. Previous distillate dewaxing technologies do not usually result in large reduction of the 85% point or the sulfur content. Such technologies employ shape-selective catalysts such as ZSM-5 which crack straight-chain paraffins and which act mainly on the front end (low boiling portion) of the feed, so that the higher boiling (back end) components, which also contain most of the sulfur, remain less affected by the treatment. Sulfur reduction of a catalytically dewaxed middle distillate feed requires additional hydrotreating which would, unfortunately degrade the octane number of the naphtha produced during the dewaxing by saturation of the olefins in this naphtha. A conventional hydrocracking process employing metals such as nickel or tungsten on an amorphous support, would not effectively lower the pour point and would reduce product yield by non-selective cracking. Naphtha produced as a by-product from hydro-cracking is also low in octane.
U.S. Pat. No. 4,390,413 (O'Rear et al) is directed to the processing of distillate and lube fractions. A intermediate pore catalyst such as ZSM-5 catalyst is used in a form which is substantially free of hydrogenation activity, under specific conditions, to dewax (remove paraffins) from petroleum by forming C.sub.3 -C.sub.4 olefins which may be further processed. Dewaxing, then distillation steps occur in separate reaction zones. A conventional hydrocracking catalyst was used.
European Patent 0189648 discloses catalytic dewaxing with an intermediate pore catalyst such as ZSM-5. The patent is directed to the production of distillates and naphtha by-products. Heavy gas oil feed is hydrowaxed. Distillation occurs following hydrodewaxing, in a separate reaction zone. Certain streams, such as heavy distillates or heavy kerosine, may be catalytically dewaxed following distillation. A conventional hydrocracking catalyst is used, having both an acidic function and a hydrogenation function.
European Patent 0316656 discloses a process for the production of high-quality gas oil from heavy feedstocks, utilizing catalytic dewaxing and desulfurization techniques. The dewaxing and desulfurization occur in different reaction zones. There is opportunity for interstage separation. Each step is optional, depending upon the characteristics of the feed. Dewaxing may be necessary but not desulfurization, or vice-versa.
British Patent Application GB2,185,753A discloses a process for the removal of waxy paraffins from hydrocarbon feedstocks which boil in the gas oil range and contain sulfur. The process comprises passing the feed over a crystalline high silica ZSM-5 variety under suitable operating conditions for the cracking of straight chain paraffins.